Unidirectional fabrics are fabrics in which the warp and weft yarns are substantially parallel and in the plane of the fabric but without the over and under crimp of a woven structure. Without such an interwoven structure, the fabric of unidirectional yarn layers must be held together by some additional structure. Examples of additional structures include resin, film, stitching, knitted fabric and woven fabric.
Unidirectional fabrics have been fabricated for a long time. For instance, U.S. Pat. No. 2,893,442 of Genin, describes laying high modulus glass threads across each other without crimping them. The threads were loosely held together by weaving with much thinner and more flexible yarn. The resulting fabric was used as reinforcement in plastic laminates.
Unidirectional fabrics may be used as a reinforcing fabric by inserting a high modulus fibre in either the weft or warp direction of knitted fabrics as the knit fabric is being formed on the knitting machine, for example, as described in U.S. Pat. Nos. 3,105,372, 3,592,025 and 3,819,461. The resulting product has unidirectional fibres in the weft or warp direction, secured in place by the knit fabric. Such fabrics are currently in production and typically used in fiberglass reinforced plastic applications. A knit fabric with a ballistic yarn inserted in either the warp or fill direction of the fabric is also known.
A second type of unidirectional fabric is used in reinforcement of composites, for example, as described in U.S. Pat. Nos. 4,416,929, 4,550,045 and 4,484,459. These fabrics generally have two or three layers of unidirectional yarns with at least two of the layers being oriented at 90 degrees to each other. Typically, two of the yarn layers are oriented either at 0/90 degrees or at 45/45 degrees to the longitudinal direction of the fabric. The yarns are then stitched together, usually with stitch lines closely spaced together, for example, at a spacing of approximately on one-eighth inch (0.3 cm). The angle at which the layers of yarns are oriented to each other may be varied and the spacing of the stitching and the length of the individual stitches may also be varied. Such a fabric was marketed by Hexcel in the 1980's as a ballistic fabric. Fabric was produced with and without a thermoplastic film between the yarn layers. With the film between the layers, the fabric was hot pressed into hard (rigid) armour. The film melted during pressing and served as the resin system in the finished composite. Without the film, the fabric was used in soft armour applications, such as vests and blankets where flexibility of the fabric is required. It is understood that the material was not widely accepted in the ballistic market.
Another type of unidirectional fabric is composed of a large diameter high performance yarn in either the warp or fill direction and a lower strength, smaller diameter yarn as the opposing yarn. By keeping the tension high in the direction of the high performance fibre, coupled with the smaller size of the opposing yarn, the high performance fibre is substantially maintained in a straight line with only minimal over and under crimp. Such fabrics are used mainly in the sail cloth industry where the fabric is fabricated into sails with the high performance yarn oriented in the direction of the load on the sail and the weaker yarn provides stability in the off-axis direction. Such fabric is usually laminated to a polyester film, the film providing some stability in the bias direction of the fabric. This fabric is also used in ballistic applications with a thermoplastic film heat laminated to one side of the fabric, for example, as disclosed in U.S. Pat. Nos. 5,437,905, 5,635,288 and 5,935,678. In ballistic applications the fabric is further processed in a second step by being cross-plied i.e. one layer is placed at 90 degrees to a second layer. The fabric is then heated and pressure is applied. The resulting two-layer fabric laminate is used in soft armour applications. Multiple layers of the material can be heat pressed to form a rigid armour laminate.
Another family of unidirectional fabrics was the subject of patents issued to Honeywell (formerly AlliedSignal), for example, U.S. Pat. Nos. 5,354,605, 5,173,138 and 4,623,574. These fabrics are produced by impregnating a unidirectional layer of filaments of high performance yarn with a thermoplastic resin system. Two layers of the resultant prepreg are cross-plied together at a 90 degree angle to form a single sheet of ballistic material. For soft armour applications, the cross-plied fabric has a thin thermoplastic film laminated to each side. For hard armour applications, the fabric is used without films and is heat laminated under pressure. These products are sold under a series of trademarks, including Spectra Shield, Spectra Flex, Spectra Shield Plus, Gold Flex, and Zyloshield.
Three dimensional fabrics may also be formed with two or more unidirectional high performance yarns oriented at 90 degrees to each other and with a high performance fibre woven into the fabric, perpendicular to the unidirectional layers. The fabric looks and performs very similar to the closely stitched unidirectional fabrics discussed above. U.S. Pat. Nos. 5,465,760, 5,085,252, 6,129,122 and 5,091,245 are directed to such fabrics.
The trend in the development of woven fabrics is to reduce the fabric crimp and spread the crossover points apart. This is accomplished by weaving yarn in a more open construction, usually retaining the plain weave construction. The individual yarns in the fabric must be flat and spread for an open construction for a ballistic fabric. Without flat, spread yarns, the interstices between the yarns become excessive and a bullet is able to slide through the resultant openings during impact, easily penetrating the layers of the armour. Improvements in yarn manufacture and weaving technology have allowed high performance yarns to be woven with little or no twist and with resulting flat, spread yarn orientation in the fabric. While the open fabric has superior performance, the decrease in weight obtained is greater than the increase in ballistic performance and more layers of fabric are required to meet ballistic specifications. The increased number of layers appears to be of benefit in and of itself. The use of additional layers is believed to distribute the impact energy more evenly throughout the layers of fabric. However, there is a limit to the openness of the weave that can be achieved with a standard woven fabric. As the openness increases, the fabric tends to become more of a scrim than a fabric, and the fabric has no merit or value in an armour application. In addition, the fabric becomes so flimsy that it can not be handled or cut without distorting the orientation of the yarns and ruining the fabric. An improvement on the woven fabric is a unidirectional fabric.
Appropriately designed unidirectional fabrics perform better in ballistic applications than woven fabrics. The weight of unidirectional fabric layers required to meet a ballistic specification is less than the weight of the layers of an equivalent woven fabric i.e. a fabric made with the same denier of ballistic yarn, required to meet the same specification. It is to be understood that different denier yarns give different ballistic results in either standard woven or unidirectional fabrics. The total weight of the finished fabric layers is used for comparison and includes any film, resin or yarn required to stabilize the unidirectional yarns.
Optimally designed unidirectional ballistic fabrics have two or more unidirectional layers of yarn at 90 degrees to each other. When more than two layers are used, the layers are alternated at 90 degrees to each other. Such orientation has been achieved by laminating two unidirectional fabrics or prepreg layers together, the top of one layer bonded to the bottom of the upper layer. This is done in a second operation using a film or resin as the adhesive layer. The 90 degree orientation is required for ballistic performance and the generally accepted standard for orientation is 90±5 degrees. Woven fabrics by their nature have warp and fill yarns oriented at 90 degrees.
A second requirement of optimally designed unidirectional fabrics is for the yarns to able to freely transmit energy away from the impact area. In order to transmit energy efficiently, the yarn must not be tightly constrained. The lack of constraint on the yarn allows the maximum dissipation of energy along the length of the yarn and will be discussed further below in the comparison of woven and unidirectional fabrics. The constraint of the fibre may be minimized by the use of a low modulus film to adhere the two layers together or use of a low strength, low tensile modulus yarn to hold the individual layers together.
Without the over and under crimp that is present in the yarns of a woven fabric, the yarns in unidirectional fabrics immediately undergo tensile stress when impacted by a projectile. In contrast, yarn in woven fabric moves backward when impacted by the projectile until the crimp is removed, and only then are the yarns in tensile stress. The backward movement of the fabric forms a depression and thus opens the weave of the fabric. The increased area of this depression reduces the number of yarns that can resist the projectile and decreases the total number of yarns directly involved in the ballistic event. Further, the cavity in the fabric formed by this backward movement limits the deformation of the projectile by constraining the sides of the projectile. This reduced area of the projectile has a further negative effect on the ballistic performance of the fabric system by restricting the number of yarns than can be behind the deformable projectile. Since the number of yarns behind the projectile is proportional to the square of the diameter of the projectile, deformation is a very important consideration in both fabric and vest designs where a deformable projectile is the threat. Further the deformable projectile absorbs energy in the deformation process. Lower deformation results in less energy being absorbed by the projectile per se.
Another reason for better performance of a unidirectional fabric, compared to a woven fabric, is that there are no points in the unidirectional fabric where the yarn is constrained. In contrast, woven fabrics constrain the individual yarns at the crossover points, particularly as backward movement under impact of the projectile tightens the fabric. Constrained points reflect the tensile wave propagated along the yarns during the ballistic event. This reflected wave is cumulative with the initial strain wave, adding to the total tensile load acting on the yarn, and prematurely breaking the yarn before the maximum amount of energy can be absorbed along its length.
The manufacture of some unidirectional fabrics has resulted in significant decreases in the weight of some vest or armour systems. However, the cost of producing the successful unidirectional fabrics is significantly more than that of a woven fabric. The increased cost is mainly due to the requirement that the individual layers of the fabric be produced in one weaving or prepreg operation and cross-plied in a second operation to produce a 0/90 construction.
Improvements in ballistic fabrics would be useful.